pixels and uploads textures on demand - many skins are never used, so
this saves some system memory)
added palette_featureflags[] for use by mdl skin loading
added TEXF_PRECACHE to all texture uploads that did not use it
added skinframe->hasalpha flag to clean up code
removed R_TextureHasAlpha function
r_glsl 1 no longer uploads fog textures
r_glsl 0, gl_combine 0 no longer loads gloss/normalmaps